Impact operated ground piercing tools designed for horizontal earth boring are well known. Such tools typically comprise an elongated torpedo shaped housing which contains an air valve system and an air-driven striker. Sudnishnikov U.S. Pat. No. 3,410,354 issued Nov. 12, 1968 exemplifies such a tool. The pressure fluid, typically pressurized air, is fed through a hose to an air inlet pipe coaxially disposed at the center of the rear of the tool. This air inlet pipe is rigidly secured to the housing and is connected or integral at the forward end thereof with a stepped bushing which is an essential part of the valve mechanism.
The striker disposed within the housing of such tools is urged forwardly against a front wall or anvil surface, and the resulting impact drives the tool forwardly into the earth. The forwardly tapered nose of the tool pushes aside and compacts earth and other obstacles to form a hole. When the striker returns to a rearward position for another stroke, friction between the outer surface of the housing and the surrounding earth prevents the tool from traveling rearwardly out of the hole as far as it was driven into the hole during the forward stroke. By this means the tool gradually makes progress through the earth.
It is also known in the art to provide such a tool with a reversing mechanism. Essentially, this amounts to changing the relative position of the air valve mechanism so that the striker no longer impacts against the front wall or anvil surface, or impacts against such front surface only lightly, and instead impacts against a rear surface, thereby driving the tool backwards out of the hole. Sudnishnikov U.S. Pat. No. 3,651,874 issued Mar. 28, 1972 exemplifies such a reversing mechanism.
The present invention relates to a finned earth boring tool, particularly one having a housing when contains a striker and an air valve mechanism, which tool moves through the earth at a greater speed as compared to a similar, unfinned tool. Prior to the development of impact operated boring tools, a wide variety of finned or bladed earth boring tools were known. Corbosiero U.S. Pat. No. 2,354,245 issued July 25, 1944 and Merrick U.S. Pat. No. 2,664,273 issued Dec. 29, 1953 exemplify such known tools. Reamers having essentially rectangular grooves in the outer cylindrical surface thereof are also known, and have been employed in vertical drilling apparatus. See, for example, Wright U.S. Pat. No. 2,498,192 issued Feb. 21, 1950 and Sato U.S. Pat. No. 3,151,687 issued Oct. 6, 1964.
A variety of finned designs have been proposed for pneumatic impact operated boring tools. The foregoing patent to Zinkiewicz, Sudnishnikov U.S. Pat. No. 4,280,573 issued July 28, 1981 and Kostylev U.S. Pat. No. 4,570,723 are exemplary of U.S. patents disclosing such designs. It is also known to embody such fins or splines in a removable headpiece which fits over the nose of the tool. See the foregoing patent to Kostylev et al. Russian Pat. Nos. 532,286 issued Apr. 7, 1981 and 658,224 issued May 5, 1979 similarly disclose boring tools having tapered fins. The foregoing finned earth boring tools are of complex construction, and the fins of such devices are difficult to replace. The fins shown in the foregoing patents are quite large and would add substantially to the overall weight of the housing, which tends to reduce the power of the tool. In rocky soils, such large fins can cause the tool to jam and stop moving.
The foregoing patent to Kostylev et al. U.S. Pat. No. 4,570,723 contains specific teachings on the use of fins in self-propelled percussion machines for driving holes. According to that patent, several structural features are asserted to be important for providing a finned impact boring device which will drive a hole straight through the ground. In discussing the foregoing Russian Pat. No. 658,224, Kostylev et al. state that the use of a single tapered section including a finned conical sleeve will tend to cause the tool to deviate from a straight line when it encounters an obstacle. Kostylev et al. also teach that having a head end section of a diameter greater than the diameter of the rest of the housing is undesirable because such small length of the thickened part of the housing fails to assure directional stability.
As to the use of fins on the housing body, Kostylev et al. teach that the diameter of the cylindrical portion of the housing is greater than the diameter of a circle described about the bottoms of the recesses between projections and less than the diameter of a circle described about the tops of the projections. This allows the cross-sectional area of the cylindrical section of the housing to be approximately equal to the cross-sectional area of the housing at the location of the recesses and projections. This feature is supposed to afford greater machine reliability without reducing the rate of hole driving because the volume of soil deformed and the diameter of the hole are maintained. The tool according to the present invention has fins disposed in a manner contrary to the teachings of the foregoing patent to Kostylev et al. as described below.